Electromagnetic (EM) geophysical surveying techniques are either natural source or controlled source. In natural source electromagnetic surveying, the response of the Earth's subsurface to natural variations in the Earth's magnetic field is measured. In the controlled source electromagnetic surveying, the response of the Earth's subsurface to externally-applied electromagnetic field is measured. Magnetotellurics (MT) is an example of natural source electromagnetic surveying. In MT measurements, natural variations in the Earth's magnetic field induce electric currents in the Earth's subsurface. Orthogonal electric and magnetic field components of the natural electromagnetic field are measured and used to determine specific ratios of electric-to-magnetic field components called tensor impedances. The tensor may be used to gain insight into the spatial distribution of the conductivity of the Earth's subsurface. Marine electromagnetic geophysical surveying typically involves deploying a plurality of multi-component acquisition apparatus on a water bottom. Each multi-component acquisition apparatus may include one or more sensors for receiving electromagnetic signals produced during surveying of the subsurface below the water bottom and electronics for recording the electromagnetic signals received at the sensor(s).
Controlled source electromagnetic surveying known in the art includes imparting alternating electric current into formations below the sea floor. In frequency controlled source electromagnetic (f-CSEM) surveying, the alternating current has one or more selected frequencies. F-CSEM surveying techniques are described, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R., 1990, An active source electromagnetic sounding system for marine use, Marine Geophysical Research, 12, 29-68. Other publications which describe the physics of and the interpretation of electromagnetic subsurface surveying include: Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes, D. C., Bone, M. N., Trigg, D. F., and DeLaurier, J. M., 1985, First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding: Geophysics 50, No. 1, 153-160; Edwards, R. N., 1997, On the resource evaluation of marine gas hydrate deposits using the sea-floor transient electric dipole-dipole method: Geophysics, 62, No. 1, 63-74; Chave, A. D., Constable, S. C. and Edwards, R. N., 1991, Electrical exploration methods for the Seafloor: Investigation in geophysics No 3, Electromagnetic methods in applied geophysics, vol. 2, application, part B, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987, On the theory of sea-floor conductivity mapping using transient electromagnetic systems: Geophysics, 52, No. 2, 204-217.
A typical f-CSEM marine survey can be described as follows. A recording vessel includes cables which connect to electrodes disposed near the sea floor. An electric power source on the vessel charges the electrodes such that a selected magnitude of alternating current, of selected frequency or frequencies, flows through the sea floor and into the Earth formations below the sea floor. At a selected distance (“offset”) from the source electrodes, receiver electrodes are disposed on the sea floor and are coupled to a voltage measuring circuit, which may be disposed on the vessel or a different vessel. The voltages imparted into the receiver electrodes are then analyzed to infer the structure and electrical properties of the Earth formations in the subsurface.
Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying (t-CSEM). In t-CSEM, electric current is imparted into the Earth at the Earth's surface (or sea floor), in a manner similar to f-CSEM. The electric current may be direct current. At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching strategies are possible; as will be explained in more detail below. Structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields. T-CSEM techniques are described, for example, in Strack, K.-M., 1992, Exploration with deep transient electromagnetics, Elsevier, 373 pp. (reprinted 1999).
U.S. Pat. No. 6,842,006 B1 issued to Conti et al. discloses a subsea electromagnetic measurement system for obtaining MT measurements of the Earth's subsurface. The system includes a central structure to which an electrode may be attached. Arms are pivotally attached to the central structure via hinges. The hinges may include a single-pin connection. The hinges may also include a wide attachment that permits vertical pivotal motion and distributes torsion over a wide area. An electrode and at least two magnetometers are coupled to each arm. The magnetometers form an orthogonal magnetic field measurement system. The arms are adapted to pivot about the hinges so that the electrodes and magnetometers rest on the seafloor when the electromagnetic system is emplaced in a selected position.